What are Volcanoes?
These iconic geologic features act as pressure valves for the planet by periodically releasing heat and molten rock from beneath the surface. While most people think of volcanoes as high, conical peaks jutting into the sky, most are in fact hidden on the seafloor, clustered in chains of seamounts, or spread along the mid-ocean ridges—where volcanic activity is greatest. They come in many other forms, as well, from chimney-like conduits that explode with lava, ash, and vapor to complex systems of cracks that spew lava at the Earth’s surface. In addition to their well-known destructive power, volcanoes are also a significant generator of new planetary crust as well as a conduit for trace elements and other materials to cycle back to the surface from the interior of the planet.
Volcanoes occur in many different parts of the world and in many different geologic settings. Most large or explosive volcanoes are found on or near subduction zones—places where one of Earth’s massive tectonic plate dives beneath, or subducts, beneath another. However, the most active volcanic activity is found along ocean ridges, places where tectonic plates are pulling apart and lava wells up from beneath the surface to fill the gaps. Many large volcanoes are also associated with mid-plate “hotspots”: huge plumes of hot mantle rock that can extend hundreds of kilometers into the depths of Earth’s mantle.
Why are they important?
The many hazards that volcanoes present—from unexpected explosions and landslides to long-term climate disruptions—make them well worth watching. The ruins of Pompeii, Italy are a graphic reminder of the hazards associated with living on or near one of the many volcanoes (active or seemingly dormant) worldwide. Nearly 1,700 years ago, people and animals living in Pompeii were almost instantaneously entombed in ash from Mt. Vesuvius. More recently, Mt. St. Helens in Washington State erupted in 1980, ripping more than 1,300 feet off the top of the mountain and causing an estimated $1.1 billion in economic damages, from deaths and injuries to destroyed bridges to lost crops.
Even seemingly inactive or dormant volcanoes may prove unexpectedly hazardous: the Mammoth Lakes region of California is home to the Long Valley caldera, which began to seep carbon dioxide and helium through cracks in the ground in the mid-1990s. The gases killed large numbers of trees in the area, and may have been responsible for some human health problems and several deaths.